Hydromechanical and Elecrohydraulic Servo system

 Mechanical-type servo valves are generally employed in the less complex systems.

HYDROMECHANICAL AND ELECROHYDRAULIC SERVO SYSTEM

MECHANICAL HYDRAULIC SERVO SYSETM

1. Application of Hydromechanical Servo Valve 

(Hydromechanical Servo System)

• Mechanical-type servo valves are generally employed in the less complex systems.

• Applications for hydromechanical servo valves are on: steering devices (such as power steering system of automobiles); test and training devices; copying devices (such as on machine tools); and heavy-duty mobile equipment.

Now we shall discuss on the automotive power-steering application.

2. Automotive Power-Steering Application of Hydromechanical Servo System

1. Construction

The construction and operation of an automotive power-steering application of hydromechanical servo system (closed-loop system) is illustrated in Fig.8.25.

2. Operation

The functioning of this hydromechanical servo system is as follows:

1. The turning of the steering wheel is the input or command signal to the servo system.

2. The steering wheel moves the valve sleeve that ports oil to the steering cylinder (actuator).

3. Now the piston rod moves the wheel through the steering linkage, as shown in Fig.8.4. 4. Since the valve spool is attached to the linkage, the valve spool moves with linkage. 5. When the valve spool has moved far enough, it cuts off oil flow to the cylinder. This stops the motion of this cylinder.

6. Thus the position of the steering wheel determines the motion of the output wheel to the desired position. To cause the further motion of the output wheel, additional motion of the steering wheel is required.

Thus a given input motion (motion of steering wheel) has produced a specific and controlled amount of output motion (motion of output wheel) and also the output motion is fedback to modify the input via the feedback line (here it is the mechanical feedback recentre called 'null'*)

• Null is the relational condition between the spool and valve port where the valve supplies no control flow at zero load pressure drop. The change in null bias resulting from changes in operating conditions or environment is called null shift.